According to conventional wireless reception technology, spatially multiplexed signals transmitted by multiple antennas are received by multiple receiving antennas in such a manner as to achieve high-speed transmission and stable reception using channel estimation techniques. This system is called “MIMO” (Multi Input Multi Out). An example of the conventional wireless reception technology is disclosed in Patent Literature 1.
FIG. 17 shows a conventional wireless transmitter and a conventional wireless receiver disclosed in Patent Literature 1. In FIG. 17, the wireless transmitter includes signal input terminal 800, space-time encoders 801 and 802, inverse fast Fourier transformers 803, 804, 805, and 806, and transmitting antennas 807, 808, 809, and 810. Space-time encoders 801 and 802 are respectively shown as “STE1” and “STE2” in FIG. 17. Inverse fast Fourier transformers 803, 804, 805, and 806 are respectively shown as “IFFT1”, “IFFT2”, “IFFT3”, and “IFFT4” in FIG. 17. Transmitting antennas 807, 808, 809, and 810 are respectively shown as “TA1”, “TA2”, “TA3”, and “TA4” in FIG. 17.
The wireless receiver includes receiving antennas 811, 812, and 813, fast Fourier transform subsystems 814, 815, and 816, space-time processor 817, space-time decoders 818 and 819, channel parameter estimator 820, and output terminals 821 and 822. Receiving antennas 811, 812, and 813 are respectively shown as “RA1”, “RA2”, and “RAp” in FIG. 17. Fast Fourier transform subsystems 814, 815, and 816 are respectively shown as “FFT1”, “FFT2”, and “FFTp” in FIG. 17. Space-time processor 817 is shown as “STP” in FIG. 17. Space-time decoders 818 and 819 are respectively shown as “STD1” and “STD2” in FIG. 17. Channel parameter estimator 820 is shown as “CPE” in FIG. 17.
In the wireless transmitter, input terminal 800 receives a data block, which is separated into a data block b1 [n, k] and a data block b2 [n, k]. The data blocks b1 [n, k] and b2 [n, k] are respectively supplied to space-time encoders 801 and 802. Space-time encoders 801 and 802 each generate two data pairs. More specifically, the data blocks b1 [n, k] and b2 [n, k] are converted into a total of four pieces of data (tm1 [n, k] to tm4 [n, k]). Inverse fast Fourier transformers 803 to 806 modulate the converted four pieces of data (tm1 [n, k] to tm4 [n, k]), and then output OFDM signals. Transmitting antennas 807 to 810 wireless-transmit the OFDM signals. The OFDM signals thus transmitted are received by receiving antennas 811 to 813. As shown in FIG. 17, the OFDM signals transmitted by transmitting antennas 807 to 810 are received in a state of overlapping each other by receiving antennas 811 to 813.
Fast Fourier transform subsystem 814 converts a signal r1 [n, k] received by receiving antenna 811 into a frequency-space signal and supplies it to space-time processor 817. Fast Fourier transform subsystem 815 converts a signal r2 [n, k] received by antenna 812 into a frequency-space signal and supplies it to space-time processor 817. Similarly, fast Fourier transform subsystem 816 converts a signal rp [n, k] received by antenna 813 into a frequency-space signal and supplies it to space-time processor 817. Channel parameter estimator 820 receives the signals converted by fast Fourier transform subsystems 814 to 816 and determines channel parameter information from the signals.
Then, channel parameter estimator 820 supplies the determined results to space-time decoders 818 and 819, so that the determined results are used for decoding. The spatially multiplexed transmission signals are separated and decoded by space-time processor 817, space-time decoders 818 and 819, and channel parameter information, and then are supplied to output terminals 821 and 822.
As an application of the spatial multiplexing demodulation technique shown in Patent Literature 1, interference suppression reception technology published by the IEICE in 2005 is shown in Non-Patent Literature 1. FIG. 18 shows a wireless transmitter and a wireless receiver disclosed in Non-Patent Literature 1.
In FIG. 18, wireless transmitter 500 includes communication controller 501, first and second IFFTs 502 and 504, and transmitting antennas 503 and 505. Its principle and operation are the same as those of the wireless transmitter of Patent Literature 1. FIG. 18 shows, for simplification, two transmitting antennas transmitting two OFDM data streams. The wireless receiver includes receiving antennas 601 and 603, first and second FFTs 602 and 604, demapper 611, Viterbi decoder 612, and interference suppressors 600. Interference suppressors 600 each include weighting/combining unit 605, transmission line estimator 606, undesired signal measurer 607, and reliability evaluator 610.
In FIG. 18, interfering station 700 transmits radio interference waves which interfere with radio waves transmitted by wireless transmitter 500 while they are passing through fading channel 900.
In the wireless receiver, receiving antennas 601 and 603 receive the OFDM signals with which the above-mentioned radio interference waves have interfered. First and second FFTs 602 and 604 fast Fourier transform the received OFDM signals and output the resulting signals on a per-OFDM-subcarrier basis. Interference suppressors 600, which are as many as the number of subcarriers, perform demodulation of the signals received from wireless transmitter 500, and at the same time, removal of the disturbing waves. In each interference suppressor 600, transmission line estimator 606 calculates a transmission line coefficient matrix “H”, which indicates the state of transmission in fading channel 900 by using the preamble of a packet. The transmission line coefficient matrix “H” is calculated by the same formula as for a normal MIMO demodulation operation. Undesired signal measurer 607 detects an interference wave signal during the time after a desired wave packet is transmitted and until the next desired wave packet is transmitted, and then calculates an inter-antenna covariance matrix Ruu.
Weighting/combining unit 605 first calculates a weighting coefficient W with which the input signals at receiving antenna 601 and 603 are combined, by using the inter-antenna covariance matrix Ruu. The inter-antenna covariance matrix Ruu is calculated on the basis of each subcarrier component of the OFDM signals and the transmission line coefficient matrix “H”. Weighting/combining unit 605 then performs a weighting and combining operation on a reception signal vector “r” using the weighting coefficient W, thereby calculating a signal vector “s”, which is obtained by W*r. The reception signal vector “r” is obtained from the input signals at receiving antennas 601 and 603. This calculation allows the demodulation to be performed with suppressed interference waves. When no interference wave exists, the inter-antenna covariance matrix Ruu contains only noise components of the interference waves. As a result, the inter-antenna covariance matrix Ruu becomes equivalent to the reception using maximum ratio combining, thus adaptively reducing reception errors all the time.
FIG. 19A shows the relation between OFDM subcarrier 701 and noise component 702 before the input signals at receiving antenna 601 and 603 are weighted and combined by weighting/combining unit 605. FIG. 19B shows OFDM subcarrier 703 and residual error 704 after the input signals are weighted and combined by weighting/combining unit 605, and then each subcarrier amplitude is normalized. From the results, it is likely that the interference wave signals remaining after noise removal are different from subcarrier to subcarrier, and that the larger the residual error, the less reliable the subcarrier. For this reason, reliability evaluator 610 calculates, as a likelihood “K” indicating the likelihood of the signal, the reciprocal of a residual error “e” (a remaining interference wave signal) at the time point when each subcarrier amplitude is normalized after the weighting and combining.
Demapper 611 restores the mapping of the per-subcarrier signals outputted from interference suppressors 600. Viterbi decoder 612 performs error correction on the signals having the restored mapping, using the likelihood “κ”, and outputs demodulated signals.
FIG. 20 shows simulation results indicating that the SIR (signal to interference ratio) required to obtain the same PER (packet error rate) can be improved by about 10 dB+5 dB by performing two processes. The two processes are interference suppression by weighting and combining, and Viterbi decoding using likelihood. In FIG. 20, the horizontal and vertical axes represent the required SIR and the PER, respectively. As shown in FIG. 20, weighting and combining effect 212 is about 10 dB, and Viterbi likelihood effect 211 is about 5 dB.
In the conventional wireless receiver, however, the channel estimation of a desired wave uses the preamble of the desired wave packet. Therefore, it is impossible to perform the channel estimation of a continuous wave with no preamble.